Abstract Type 2 Diabetes and Sexual Orientation Disparities in Women type 2 diabetes (T2D) is a critical public health problem affecting the U.S. because it causes enormous morbidity, disability, premature mortality, and financial and social costs to society. Lesbian and bisexual (LB) women may be at elevated risk for developing T2D because they are more likely than heterosexual women to experience obesity and other risk factors linked with T2D such as cigarette smoking, violence victimization, and depressive distress. Nonetheless, knowledge of T2D and how it may disproportionately affect LB women is severely limited. Studies using longitudinal designs that have comprehensively examined how lifestyle, diet, and psychosocial risk factors for T2D may differ between LB and heterosexual women across the life course are virtually nonexistent. In addition, evidence to understand how risk for developing T2D may differ between LB and heterosexual women is inconclusive and limited to four cross-sectional health surveys. To our knowledge, no studies with longitudinal designs have investigated how incidence of T2D may differ between LB and heterosexual women. Furthermore, analytic epidemiological studies examining how sexual-orientation disparities in risk factors for T2D may contribute to LB women's disparities in occurrence of T2D are nonexistent. Consequently, huge knowledge gaps exist in understanding the extent to which T2D and related risk factors may disproportionately affect LB women. According to the IOM report The Health of Lesbian, Gay, Bisexual, and Transgender People: Building a Foundation for Better Understanding, LB women face large health disparities, and understanding these disparities and how to eliminate them is hampered by methodological complexities involving research with this population. To address these problems and gaps in the literature, we propose to carry out a rigorous quantitative study using prospectively collected data spanning more than 20 years (1989 through 2011) from nearly 100,000 women participating in the Nurses' Health Study II. Our study will compare incidence of developing T2D among LB women to that among heterosexual women, compare risk factors for T2D in multiple domains (e.g., diet, lifestyle, psychosocial) and over the life course (i.e., from childhood through age 65 years) among LB women to that among heterosexual women, and examine how lesbian and bisexual women's disparities in risk factors for T2D prospectively explain their disparities in risk for developing T2D. Our study has high potential for public health impact because findings will increase understanding of: 1) the extent to which risk factors for T2D disproportionately affect LB women over a substantial portion of the life course, 2) the extent to which incidence of T2D disproportionately impacts LB women, and 3) potential social and behavioral factors that explain sexual-orientation-based disparities.